Respiratory nitrate reductase complexes will be studied in Staphylococcus aureus, a facultative fermenter, and in the denitrifier, Paracoccus (Micrococcus) denitrificans. The emphasis will be upon the role of membrane cytochromes in the formation, assembly and function of the complexes. The relation between the cytoplasmic and membrane associated forms of nitrate reductase in S. aureus will receive particular attention.